Technical Field
The present disclosure relates to communication using orthogonal frequency division multiplexing (OFDM) for communications, and more particularly, to a multiplexing method.
Description of the Related Art
Recently, in addition to existing mobile phones, other wireless communication devices have been developed. It is estimated that by the year 2020, the demands on wireless communication resources will be one thousand times greater than the current wireless communication demands, which creates a need for new wireless communication resources, which can be limited by restrictions within existing wireless communication technologies. Within the 3GPP international standard for wireless communication, a fifth generation (5G) standard has been developed, which further increases frequency utilization efficiency in order to accommodate the increasing demands on wireless communication systems.
In the present LTE/LTE-A, orthogonal frequency-division multiplexing (OFDM) is employed, and multiplexing is performed using orthogonality in the frequency domain. Further, carrier aggregation is used to increase wireless capacity by broadening bandwidth but does not solve the problem of radio wave resource depletion.
In some cases, the modulation order per subcarrier in OFDM is increased in order to increase a total number of multiplexed symbols. For example, LTE uses 16QAM (quadrature amplitude modulation) and LTE-A uses 256QAM. In the case of LTE-A, the total number of transmitted bits per subcarrier increases to eight, but the distance between symbols decreases so that a higher energy per bit to noise power spectral density (Eb/N0) is required to maintain a predetermined physical layer bit error rate (PHY BER).
Other multiplexing methods have been introduced to solve the problems introduced by increasing the number of modulated bits per subcarrier that include employing orthogonality relationships between the modulated symbols that include frequency, time and code. For example, OFDM is associated with orthogonal frequencies, and CDMA is associated with orthogonal codes. In what is now considered to be fifth generation (5G) LTE, a filter bank multi carrier (FBMC) modulation method is used, which employs Wavelet OFDM. FBMC modulation makes a cycle prefix (CP) unnecessary by using time orthogonality and frequency orthogonality together. In addition, a guard band is also unnecessary because out-of-band radiation is very low. FBMC modulation allows a larger number of user equipments (UEs) to use identical bandwidths; however, the amount of improvement gained by eliminating the CP and guard band is at most ten percent, which is not enough to accommodate the increased demands on wireless communication systems.
In addition, non-orthogonal multiple access (NOMA) has also been considered for solving the problems resulting from increased demands on wireless communication systems. NOMA allows multiplexed signals to be separated at a receiver based on differences in signal-to-noise ratios (SNIRs) and allows transmitted signals to be multiplexed based on amplitude. For example, two signals can be multiplexed into a first layer and a second layer, and the transmitting power of the multiplexed signal is determined based on achieving a SNIR for the first layer and the second layer at the receiver that satisfies a desired BER. At the receiver, the multiplexed layers are sequentially recovered by subtracting a demodulated signal from the received signal. NOMA allows for greater amounts of multiplexing, but lower signal layers cause interference with the higher layers, which causes an overall increase in transmit power, which can cause difficulties for terminal devices with limited transmit powers. For example, two to three multiplexed layers are considered practical at this time.
In addition, NOMA uses a so-called adaptive modulation method to keep the transmitting power in a cell or terminal device constant. For example, adaptive modulation allows the amount of modulation to be changed based on the SNIR at the receiver. However, communication speeds can be very slow with adaptive modulation depending on the type of modulation being used, and frequency utilization efficiency can become even worse.